Peter, Walter, Olivia and Broyles pursue a strange and deadly occurrence in Philadelphia where a bomb blew up inside a train station but left no trace of any explosive device.

The perplexing and unexplained set of circumstances returns Walter to the lab to closely examine the human remains where he uncovers an unlikely energy source that triggered the explosion. With the explosive threat of more bombs and links to a classified military project, the intense investigation leads Olivia and Peter to Iraq. Fringe case file # 203-10109

Goof: As Dunham stands to brief the room regarding Burgess while seated, pushes herself out from her desk with her cane facing one way, then pushes herself up with the cane having switched to the opposite angle when the shot changes.

Trivia: 47 reference: Walter states he stopped counting at 47 needle punches in a victim. 47 is often referenced in the Star Trek shows, and the tradition carried over to Fringe from J.J. Abrams and others.

Observer Sighting--The Observer receives and reviews photographs of Walter from a secretive courier.